One type of thermal printer employs a dye-donor element placed over a dye-receiver element. The two elements together are moved past a print head having a plurality of very small heat "sources". When a particular heating source is energized, thermal energy from it causes a small dot or pixel of dye to transfer from the dye donor element onto the receiver element. The density of each dye pixel is a function of the amount of energy delivered from the respective heating source of the print head to the dye donor element. The individual pixels are printed in accordance with image data; all of the dye pixels thus formed together define the image printed on the receiver element.
Because light from a laser can be focused to an ultrafine, intense spot of heat power and can be modulated at very high speed, lasers (such as small, relatively inexpensive diode lasers), are now the preferred heating sources for printing the dye pixels in the more advanced thermal printers. But in the case where pixels are printed at very fine pitch on very closely spaced lines (e.g., 1800 lines per inch and 1800 pixels per inch), it becomes impracticable to provide an individual laser for each line across the width of a page being printed. For example, a 10 inch wide page would require 18,000 lasers along with their respective drive circuits! On the other hand, using only one laser and scanning the lines across a page in sequence to print an image is a very much slower operation than when multiple lasers are used.
In U.S. patent application Ser. No. 451,655, filed Dec. 18, 1989, entitled "Thermal Printer", and assigned to an assignee in common with the present patent application, there is disclosed a thermal printer employing a plurality of lasers for printing a like plurality of lines of print pixels at the same time. This thermal printer produces full color pictures printed by thermal dye transfer in accordance with electronic image data corresponding to the pixels of a master image. The pictures so produced have ultra-fine detail and faithful color rendition which rival, and in some instances exceed in visual quality, large photographic prints made by state-of-the-art photography. This thermal printer is able to produce either continuous-tone or half-tone prints In the continuous tone mode, the ultra-fine printed pixels of colored dye have densities which vary over a continuous tone scale in accordance with the image data. On the other hand in the half-tone mode, the ultra-fine print pixels which define the picture are formed by more or fewer micro-pixels of dye such that the pixels printed closely together appear to the eye as having greater or lesser density and thus simulate a continuous tone scale. Half-tone, offset printing is widely used for example, in printing and publishing. It is common practice in this and related industries first to obtain and visually inspect "proof" prints prior to production so that any visual blemishes, artifacts of the half-tone process, or other undesirable qualities in the "printed" pictures, (which would otherwise occur in production) can be corrected before production begins In the past, the obtaining of these "proof" prints has involved considerable time delay and significant extra expense. This thermal printer, by virtue of its unique design and mode of operation is able to produce quickly (within minutes) an authentic half-tone printed image which (for all intents and purposes) is visually indistinguishable from the highest quality color image made by offset printing. And by comparison, the initial setup costs and processing times for the printing plates required in high quality offset printing are many times (e.g., hundreds) the costs and times required by this thermal printer to produce "proof" prints of equal quality. This not only simplifies the publishing operation prior to production, but helps a publisher improve the visual quality of the end product (e.g., an illustrated magazine).
The human eye is extremely sensitive to differences in tone scale, to apparent graininess, to color balance and registration, and to various other incidental defects (termed printing artifacts) in a picture which may occur as a result of the process by which the picture is reproduced. Thus it is highly desirable for a thermal printer such as described above, when used in critical applications, to be as free as possible from such printing artifacts.
The thermal printer described in the above-mentioned U.S. Patent Application has a rotating drum on which can be mounted a print receiving element with a dye donor element held closely on top of it. The two elements are in the form of thin flexible rectangular sheets of material mounted around the circumference of the drum. As the drum rotates, a thermal print head, with individual channels of laser light beams in closely spaced, ultra-fine light spots focused on the dye element, is moved in a lateral direction parallel to the axis of the drum. With each rotation of the drum, multiple lines (termed a "swath") of micro-pixels are printed on the receiving element in accordance with line data applied to the electronic driving circuits of the respective laser channels. There are as many lines printed in a swath as there are laser channels (for example, 20 lines with a lateral spacing of 1800 lines per inch) and there are as many swaths (with "seamless" or invisible over-lapping) as required to print an image of a given page width. It has been found, however, that even minute differences in the light output intensity levels of the individual lasers (where supposedly equal light intensities are called for by the same image data) can cause objectionable visual differences in the densities of the micro-pixels printed by the individual laser channels. Light power differences as seemingly unimportant as a small fraction of one percent of a desired laser output power level are visually noticeable as density variations of the printed pixels. It is highly desirable to be able to quickly and easily adjust precisely the light intensity of each laser so that all of the laser channels, for a given data command, print uniformly.
Semiconductor diode lasers, such as used in the thermal printer described above, operate (when on) at relatively high temperatures. Though the lifetime of such a laser is relatively long (thousands of hours), because of operating environment and other factors, a laser's light output power changes (sometimes erratically) as ageing progresses. Thus even though the lasers in a given printer are initially adjusted to give exactly the same light power output level, because of subsequent ageing of the lasers, their individual light power levels inevitably change slightly resulting in cumulatively diverging power levels. These cumulative changes, even though slight, degrade the high quality of the images printed. Thus, at some point, it becomes necessary to re-adjust and accurately calibrate some or all of the lasers so that they again print within a desired degree of uniformity (e.g., a small fraction of one percent). In the past such re-adjustments and calibrations have been tedious and time-consuming operations. It is doubly difficult in the field where a thermal printer is in use. Moreover, the problem is compounded in a thermal printer which employs a larger rather than a fewer number of lasers so that more lines are printed at once and the complete printing of an image takes less time.
The present invention provides a simple, automatic, and highly cost effective solution to this problem of keeping the multiple lasers of a thermal printer operating with near perfect uniformity throughout their lifetimes.